Cyclophosphamide, a nitrogen mustard derivative, is currently being evaluated as an immunosuppressive agent in patients with SLE and nephritis. Although it is now administered as an intermittent intravenous bolus (0.5 - 1.0 gm/m squared, oral therapy on an outpatient basis would be preferable. The bioavailability of CTX at the high doses used is unknown. We have developed a specific and sensitive gas chromatographic assay procedure using nitrogen-phosphorus-sensitive detector to quantitate CTX is biological fluids. Briefly, samples and internal standard (isophosphamide) are extracted with ethyl acetate, derivatized using trifluoroacetic anhydride and assayed by gas chromatography. The bioavailability of high-dose CTX will be studied in 5 SLE patients by analysis of timed plasma samples following either intravenous or oral CTX and comparison of the areas of the resulting plasma concentration-time curves extrapolated to infinity. Urinary excretion of CTX will also be monitored.